The present invention relates to a semiconductor integrated circuit device, and more particularly to a monolithic semiconductor integrated circuit device including an audio signal processing circuit.
When circuit elements of a circuit dealing with an audio signal, for example a filter circuit, are incorporated in a semiconductor integrated circuit, a time constant of the filter circuit has manufacturing variations. For example, the resistance value of a resistor element has a variation of about .+-.25% and the capacitance value of a capacitor has a variation of about .+-.30%. As a result, the time constant of a time constant circuit including such resistor element and capacitor has a large variation of about .+-.60%. Especially, in the case where two or more filter circuits having such large variations are involved in a semiconductor integrated circuit, the possession of the above-mentioned variation by each filter circuit does not give assurance that no reliability for the electric characteristic of the overall system is lost.
A basic circuit for a filter has been disclosed in, for example, "PRACTICAL ELECTRONIC CIRCUIT HANDBOOK (2)" published by CQ Publishing Co., Ltd. on Oct. 20, 1975, pp. 281-289. In the active filter circuit, time constant circuit elements including a capacitor C and a resistor R are constructed by external parts in order to suppress the variation of a cut-off frequency of the filter circuit.
On the other hand, in order to improve the high-frequency performance of a transistor included in an integrating circuit, JP-A-63-193710 (laid open on Aug. 11, 1988) has proposed to make an emitter current of the transistor as large as possible for the purpose of increasing a transition frequency of the transistor. The proposed integrated circuit includes a differential amplifier and is provided with a terminal for supplying a signal which controls the emitter current of a transistor forming the differential amplifier.